This invention relates to liquid scintillation counting compositions, and more particularly concerns compositions for counting aqueous radioactive samples.
The use of liquid scintillation counting systems including an aromatic hydrocarbon solvent, an ethoxylated alkyl phenol surfactant, and a scintillation solute, has become a popular method for counting aqueous radioactive samples. The original sample is dispersed in the scintillation counting composition, and the emitted radioactivity is converted by the cintillation solute to light flashes (fluorescence). These pulses are of an intensity proportional to the energy of the radiation, and occur at a frequency proportional to the disintegrations per unit time occurring in the sample. Instrumental detection, recording, and analysis of the pulses thus affords a measure of the radioactivity of the initial sample.
Inasmuch as the light detector is responsive to light from any source, it is evident that all other illumination must be excluded when the sample is being counted. Thus, if spurious light is produced when the counting composition is mixed with the sample (luminescence) or excited with light or heat (phosphoresence), false pulses are recorded and the analytical result is compromised.
Conventionally, phosphoresence is minimized by counting the sample in the dark and at low temperature, e.g. 6.degree. or 7.degree. C. Luminescence gradually decays, and is commonly minimized by storing the sample for several days before counting. Low temperatures, however, tend to reduce the dissipation rate of chemically induced luminescence (chemiluminescence).
The problem of chemiluminescence is particularly severe with alkaline radioactive samples. Many common sample materials, e.g. body fluids, are inherently alkaline, while other samples such as animal tissues are frequently dissolved in alkaline media. Alkaline chemiluminescence can readily produce a false light count (or background) of over 1.5 million disintegrations per minute (DPM); an acceptable background should not exceed 50 DPM. This background problem is especially serious with biological samples, where the radionuclide emits low energy beta particles, and may have a low absolute activity.
The problem of avoiding chemiluminescence in liquid scintillation counting compositions has received considerable attention. It has been suggested that the composition be pre-treated with an alkaline medium before an akaline sample is added; this has not proved effective. Alternatively, it has been proposed to include an acid such as hydrochloric acid in the composition to react with any alkaline materials; while effective, this procedure dilutes the sample, introduced additional water, and gives lower counting efficiencies as a result of the presence of acid. Some researchers, believing that chemiluminescence is produced by peroxides, have recommended the inclusion of antioxidants or pre-reduction of peroxides with various chemical reducing agents; antioxidants, however, produce undesirable color formation (color quench), and pre-reduction of peroxides with existing procedures forms gas bubbles and insoluble salts.
Accordingly, one object of the present invention is to provide, for the counting of aqueous radioactive samples, a liquid scintillation counting composition of the aromatic hydrocarbon-ethoxylated alkyl phenol surfactant type, which reduces, if not eliminates, chemiluminescence upon the addition of an alkaline material to the composition.
Also associated with the use of aromatic hydrocarbon-ethoxylated alkyl phenol type scintillation counting compositions is the problem of water miscibility. Ideally, an aqueous sample should mix easily into the aromatic hydrocarbon and ethoxylated alkyl phenol to provide a homogeneous solution or emulsion. In present practice, a single phase clear solution is obtained only up to about 15% water by weight. At higher water concentrations, up to about 25-30%, the mixture forms an unstable two-phase material, while at still higher water contents the product is a viscous gel. (These ranges vary with the specific composition and the specific temperature; the best commercial liquid scintillation counting composition produces two phases at 15-30 weight % water at 21.degree. C, or between 13 and 25% water at 6.degree. C.) Two-phase counting compositions tend to be somewhat unstable with respect to time, while gels are resistive to homogeneous dispersions of the radioactive substances and are unsuitable in flow systems.
Accordingly, another object of the invention is to provide an aromatic hydrocarbon-ethoxylated alkyl phenol type scintillation counting composition which, when admixed with an aqueous sample, produces a single phase, low viscosity, composition over a wide range of water contents and temperatures. Otherwise stated, an objective is to enhance the water miscibility of aromatic hydrocarbon-ethoxylated alkyl phenol type counting compositions.
Other and further aims, objects and advantages of the invention will become apparent as the description thereof proceeds.